Question - I have doubled letters down the page. Replacing the cartridges did not help.
Answer The most likely cause is ink buildup on the encoder strip. The web page HP had on this subject has been removed, but said something like the following. Read and heed the warnings - the encoder strip is fragile and can be damaged with improper handling. Do not use any solvents or cleaners other than water.
HP Deskjet, HP Business Inkjet, and HP Color Inkjet Printers - Cleaning the Encoder Strip for Vertical Flaws in Print, Margin Offsets, or "Stair Stepped" Margins
The encoder strip is the semi-clear strip located just behind the carriage rod. Its purpose is to help the print cartridge accurately keep track of its position as it moves along the carriage rod. Sometimes dust, ink, or oil can accumulate on this strip and cause the following problems:
Prints "stair step" down the left margin of the self-test page.
The left or right margins of the self-test are not properly placed.
The ink cartridge carriage slams to the left side of the printer.
The self-test shows double-printed or shadowed letters.
Flaws run down the length of a printed page (portrait orientation), or across the page (landscape orientation).
Printing stops, and the control panel lights on the printer indicate an error.
Gaps appear in printed text or images.
Do not clean the encoder strip for the following symptoms: media pick issues, paper jams, paper skewing, ink-cartridge rejection, missing dots or white streaks through print, and smeared or streaked black ink.
Clean the encoder strip inside the printer by following the steps below:
Extreme care must be taken during the cleaning process. If too much pressure is placed on the encoder strip, it could become scratched (destroying the etching on the strip), bent, broken, or even become dislodged from the inside of the printer. If damage of this type occurs, the printer must be returned to HP for repair.
With the printer turned on, open the top cover to allow the print cartridge cradle to move to the center of the printer.
Unplug the power cord from the printer.
Slide the cradle to the far-right position.
Identify the semi-clear encoder strip, located just above the round, metal, cradle rod.
Figure 1: The semi-clear encoder strip.
When pulling the cloth along the encoder strip in Step 5, grasp the strip gently, to ensure that the strip does not break. Pull the cloth only in a right-to-left direction, otherwise there is risk the encoder strip may become dislodged from its mounting tabs.
Moisten a clean cloth or tissue with water, then gently grasp the strip between the thumb and forefinger, and carefully pull the moistened cloth along the entire length of the strip in a right-to-left direction. Repeat this step with a different portion of the cloth to ensure that all foreign matter is removed.
Move the cradle to the far-left position and repeat Step 5 on the right side of the encoder strip, beginning as far right as possible. Again, gently pull the cloth in a right-to-left direction.
Allow the encoder strip to dry, or gently dry the encoder strip with a clean, dry cloth or tissue.
Position the cradle near the center, and then plug the power cord into the printer.
Close the top cover to allow the print cartridge cradle to move back to the right of the printer.
Print a self-test to ensure that the printer is functioning properly.